The present invention relates to a device for maintaining the switching state interrupting the anode-cathode current of a thyristor that can be turned off ("GTO Thyristor") also in the event of a failure of the negative control voltage causing this switching state between the gate and cathode terminal of the thyristor.
GTO (gate turn-off) thyristors, as well known, are distinguished in contrast to conventional thyristors by the provision that the anode-cathode current can be interrupted by the component any time in normal operation by applying a sufficiently negative gate-cathode voltage. The component is thereby switched into the current-blocking switching state without the need to wait for a natural zero crossing of the anode-cathode current. This switching state is preserved in principle also if the cutting-off negative gate-cathode voltage disappears in the meantime, i.e., if the gate terminal is "open". As long as maximally permissible values for the occurring static anode-cathode voltage or its rate of change are not exceeded in the cutoff state, undesirable "head-over firing" of the GTO thyristor need not be expected.
Limits of this kind, however, may be subject to variations which are heavily temperature-dependent and vary from unit to unit. Furthermore, overvoltages with a high rate of rise which frequently act from other circuit parts on the anode-cathode path of the cutoff thyristor cannot be precluded with certainty. For these reasons and because of the need for optimum component utilization, the cutoff behavior must be improved and its maintenance assured under all operating conditions. In the normal operation of a circuit equipped with GTO thyristors, this can be achieved in a simple manner by the provision that the turning-off gate-cathode voltage continues to act on the control path of every thyristor also in its cutoff state. The thermal cut-off current which flows into the thyristor anode in the cut-off state and which can cause with corresponding static and/or dynamic voltage stress of the anode-cathode path, via the so-called "avalanche effect", the head-over firing of the thyristor is conducted off by the applied negative gate-cathode voltage.
In the case of a defect on the other hand, i.e., in the event of a failure of the negative gate-cathode voltage maintaining the cut-off state or in the event of the failure of supply voltages which drive this control voltage indirectly, there is again the danger of head-over firing. Especially in converter circuits, for instance, in inverters, also the power section is heavily affected by short circuits in addition to the causing failure of one or several supply voltages in the control section.
To assure the cut-off behavior of GTO thyristors, especially in the event of a failure of the negative gate-cathode control voltage, it is already known to wire the gate-cathode path with a low-resistance resistor. A device of this type is shown in FIG. 1. There, the resistor acts as a shunt 24 and conducts the thermal cut-off current --i.sub.G coming from the gate terminal GT past the npn transistor structure internal to the thyristor directly to the external cathode terminal K.
However, it is a considerable disadvantage of this device that this resistor causes a considerable power loss especially during the turning off and, in the cut-off state of the thyristor due to the applied negative gate cathode voltage. In addition, the shunt is more effective, the lower its resistance. Thus, considerably more driving power must be made available which causes great expense especially if switching network parts are used. In addition, additional measures are required for removing the heat occurring thereby in the control circuit.